the best nailing pattern for built-up beams?
A. The critical issue with
built-up beams is that all the layers must deflect together and
by the same distance in order to be properly sharing the load.
For beams where the load comes down evenly on top of the beam,
such as drop beams or beams directly under bearing walls, the
nailing pattern is not all that critical. All you need are
enough nails to hold the layers together and keep them from
twisting. For beams loaded from the side, however, and
especially for beams loaded from one side only, the nailing
pattern is critical.
When beams are loaded from the side, there must be enough
nails to transfer the load through the loaded member and into
the attached members. For example, if a beam consists of three
2x10s loaded from one side only, the loaded member should only
carry 1/3 of the weight. To transfer the rest of the load into
the attached members there must be enough nails from the loaded
2x10 into the center 2x10 to transfer 2/3 of the load, and
enough nails from the far side 2x10 into the center 2x10 to
transfer the final 1/3 of the load into that outer member.
These numbers assume that all three 2x10s rest fully on the
supports; the situation gets more complicated when the members
are not all the same size or material. The bottom line, though,
is that if all the pieces deflect together and equally, the
beam should perform as designed.
At a minimum, I recommend pairs of 16d nails every 12 inches
along the beam, with the top row of nails 1 1/2 inches or so
from the top of the beam, and the bottom row 1 1/2 inches or so
up from the bottom. Use the same nailing pattern on both sides
for triple beams, and check with an engineer whenever you think
the loads involved might be unusually heavy.—C.D.